Electrochemical impedance spectroscopy (EIS) has been used in studies of electrode and plate behavior during charging and discharging for years. The impedance response of the battery, or more broadly, any electrochemical energy storage and/or conversion device (including certain types of fuel cell technologies, solar cells, and even certain types of capacitor technologies), depends on the measurement frequency and the “state” of the energy device at the time it is tested. For the specific case of a battery, this method has been reported to be affected, to varying degrees, by many fundamental battery parameters, including battery design, manufacturing tolerances, aging, temperature, and state-of-charge (SoC). The goal of EIS State-of-Health (SoH) monitoring techniques is to extract as much information as possible about the electrochemical health of the battery. Based on the variability introduced by the parameters noted above this monitoring technique has fallen short in the market place. The evolution of the technology followed a very predictable path and this application discloses the next generation of this technology which is a revolutionary break-through in the science of EIS SoH battery monitoring.
This disclosure will elucidate the novelty, inventive step and industrial viability of a method and device for more accurately measuring the SoH of an energy device by illustrating the drawbacks of the current SoH measurement and prediction techniques on lead acid battery chemistry and showing how using ultrasonic energy as part of the measurement process overcomes these deficiencies. However, it should be understood that the disclosures here can be applied to any electrochemical energy storage and/or energy conversion devices (generally referred to simple as “energy devices”), including but not limited to, certain types of fuel cells, solar cells and capacitor technologies. In the broad understanding of the method and device disclosed here, it applies to any electrochemical energy device in which there are ionic species kinetic limitations and/or deficiencies. Certain fuel cells, solar cells and capacitors also have known ionic kinetic deficiencies which impact their operating characteristics and performance and it should be understood that this method and device disclosed encompasses these type of devices as well.
A brief background is necessary in order to elucidate the novelty of the invention being presented.
In recent years there has been considerable activity and debate regarding the use of internal “impedance” characteristics as a battery condition measurement. The interest reflects the desire for simple electronic means to replace discharge testing as a practical determination of residual battery capacity (i.e. both SoC and cycle life), particularly given the increased usage of valve-regulated lead-acid (VRLA) batteries (for the lead acid chemistry example). The advanced electronic monitoring technique disclosed here, in combination with the internal impedance degradation control methodology disclosed in U.S. Pat. No. 7,592,094, provide some powerful tools for battery monitoring and management systems. Utilization and effective integration of these mechanical excitation techniques separately or in combination will result in a technology platform making possible unprecedented improvements in the utilization of energy storage devices (i.e. the battery—Li-ion, Lead Acid, NiMh, Ni-based, Zn-based, etc.) while also dramatically improving the system's overall performance, lifetime and safety. In general, the currently available battery State-of-Charge (SoC) monitoring and charge control systems do not adequately utilize energy storage systems to their fullest potential mostly based on an inability to empirically measure and thereby compensate for the negative effects of a battery's internal impedance build-up over service life (i.e. degradation). In fact, much of today's art practices in this field intensify the decline in battery performance and accelerate its failure rate by not taking into account and addressing the inherent inefficiencies and physiochemical differences brought on by the increasing battery internal impedance over service life. Furthermore, as battery-based systems become more and more demanding (i.e. more frequent charge and discharge, faster and more rapid transients, larger amplitudes, etc.), for applications, such as hybrid electric vehicles (HeV), hybrid electric vehicles (PHeV), electric vehicles (EV), grid scale frequency and voltage regulation, smarter smart phones, etc., this negative effect and trend on battery performance and service life will worsen.
As an example of how a build-up of a battery's internal impedance impacts its performance is shown in FIG. 7. FIG. 7 is data collected from performing cycle life testing of a commercial off-the-shelf Lead Acid (LA) battery. The plots shown are from the second of two 4000 minute lifetime cycle tests where the Open-Circuit Voltage (OCV) (after 20 seconds rest time) was measured until the charging cell reached 2.04V and then switched to discharge. Once the Closed-Circuit Voltage (CCV) discharge voltage reached 1.7V the next charge sequence would commence, effectively cycling the cells between 1.7V and 2.04V. The mechanical excitation delivery device/system was at a 100% duty cycle during charge and discharge and operated at 3.26 MHz.
In FIG. 7, as the battery ages (i.e. declines in health as cycle life increases) the measured charge CCV (i.e. VBattery) continues to rise and it takes longer and longer for its OCV (after 20 second rest period) to reach a threshold of 2.04V. This rise in CCV is caused by a steady but degrading build-up of the battery's internal impedance. It is also shown that as the battery ages the difference between the initial CCV at the end of the first few charges and that of the highest CCV taken at the end of the final charge is much greater for the non-vibrating cell with no mechanical excitation applied, than for the vibrating cell. This is a clear indicator that the mechanical excitation substantially minimizes the rate (i.e. the slope of CCV vs. cycle time) at which this degrading internal impedance (ZInt) builds up. It is worthy to note that at around 3600 minutes (really, 7600 minutes because this is second 4000 minute test) into the cycle testing the non-vibrating cell commenced a charge and would not switch over to discharge. This was the final cycle of the non-vibrating cell whereby it failed to accept charge energy, i.e. this signifies a battery failure which is the end of the battery life. The test for the cell with mechanical excitation (vibration) lasted many more cycles.
As another example, FIG. 8 is a Cyclic-Voltammetry (CV) test which compares the Current (I) vs. Voltage (V) response of a 1 cm2 area of porous lead (Pb) electrode submerged in sulfuric acid at a 1.31 specific gravity at 80° F. with and without mechanical excitation applied and at the same scan rate. The mechanical excitation delivery device/system was at a 100% duty cycle during charge and discharge scans and operated at 6.5 MHz. The current vs. voltage profile for the non-vibrating (i.e. control cell) and vibrating (i.e. test cell) are shown with legends in the figure. The area under the vibrating cell's discharge curve (which peaks at around 1.05V) is about 75% greater than the corresponding area under the non-vibrating cell's discharge curve (which peaks at around 1.13V). The area under these curves represent both charge (area under upper curves, positive region) and discharge (area under lower curves, negative region) power (i.e. Power=V*I), and therefore, can be directly correlated to charge energy in and discharge energy out (i.e. A-hr capacity—run time) based on using the same scan rate. The charge energy in to the control cell was larger than the charge energy in to the test cell, and the discharge energy out of the test cell was larger than the control cell. Therefore, the test cells electrochemical conversion efficiency was much higher than that of the control cells, on the order of 2×. Thus, the addition of high frequency mechanical excitation to this Pb-acid electrode system completely altered its electrical response characteristics. Given this, batteries with the proper mechanical excitation applied will behave much differently from what the industry is accustomed to and it will be important for battery manufacturers, integrators and users not to rely on the electrical response characteristics that are typically associated with the battery chemistry make-up alone. It should be noted that there are also other physiochemical benefits demonstrated and shown from this CV testing which provides further objective evidence that the test (i.e. vibrating) cell's internal impedance was significantly reduced.
It is widely accepted that the only way to truly know the true health of a battery is to periodically conduct test discharges. Once the test discharge is complete, and passes, this is a ‘good’ indicator that the battery is operating as designed and it provides peace-of-mind that the battery will perform its intended function when called upon. Unfortunately, conducting a test discharge is costly and time consuming. More importantly, if a test discharge is run on a battery pack too often it will ultimately reduce the battery's operating life, and the subsequent replacement will prove to be extremely costly. Batteries do not respond well to being fully discharged and it directly takes away from the battery life.
When conducting a test discharge it is often necessary to take the battery off-line, i.e. disconnect from its intended function, and therefore alternative measures for back-up power need to be in place. Typically, a back-up battery pack, or other means of power, such as diesel generator, fuel-cell, etc., is added to the system design to minimize the impact of this necessary operational restriction, however, this brute force attempt adds significant up front capital and maintenance cost to the project. The industry is in need of an on-line electronic measurement technique that will supply the user accurate SoH information in real-time based on empirical data acquired from a battery itself. If one does not use real-time empirical SoH date, or uses prediction algorithms or look-up tables such as are often supplied by battery or device manufacturer or even worse yet, if it is not accounted for at all, then the systems which monitor, control and manage the energy storage systems performance, lifetime and safety are compromised. Simply put: there is no substitute for real-time empirical state of health data obtained from the battery itself, as the basis for knowing the actual state of charge, and thereby being able to accurately report data to the user as well as monitor and control and optimize the battery. The length of a test discharge for a primary battery back-up plant is dependent on the rate at which it is discharged. Typically, this time is determined by how much of a load is on the system and how long it may be needed in an emergency situation. As a point of reference, some applications require the battery pack to support critical loads for a period up to ˜72 Hours. Therefore, the minimum time the battery pack will be off-line is approximately 3 days, while the battery is being discharged. However, after the 3-day test discharge is complete the battery is still not available for its intended function. The battery must be charged back up and this is also a time consuming process. If the battery is charged too hard or too fast it will take away from the battery's operating life, and again the replacement costs are significant. Thus, a general rule is that the battery should be charged for at least the length of time it was discharged, however, battery manufacturers would prefer a much longer charge time (on the order of 30 days for very large lead-acid battery packs). Therefore, the minimum time the critical battery back-up source in this example will be off-line, excluding set-up and testing time, is approximately 6 days. However, this aggressive charging algorithm is on the edge of what battery manufacturers are willing to warrantee, so prudent engineering judgment would dictate using longer charge times, thus increasing the down time of a critical energy source.
A purpose of this novel, inventive and industrious mechanical excitation technique is to remove this inherent uncertainty in the electronic measurement technique, thereby obviating the need to conduct frequent and costly test discharges. Conducting periodic test discharges of the plants critical battery back-up not only places sever operational restrictions on the plant, but, even more importantly it reduces the operating life of the battery pack and results in premature battery pack replacement. If the utility tries to minimize the length of time that this necessary operational restriction is in place, by charging the battery at a faster than recommended rate, than this will ultimately result in unwanted and premature costly battery pack replacement. Thus, the peace of mind that comes with knowing that your critical battery back-up source is ready for its intended function comes at a great cost. The utility has to weigh the risk of minimizing operational restrictions against the looming cost of replacing its battery pack. As a result, many companies are attempting to determine the battery health without test discharging. The method disclosed herein will offer a piece of mind to the plant owners that there battery pack will perform its intended function without the need to perform costly, restrictive and timely test discharges.